The Transport Accident Safety Commission says more freight trains could derail because of worn-out parts.
Three trains were derailed between 2008 - 2009 due to broken bogie side frames.
A report by the commission says the bogies, which are chassis connected to the wheels, had cracks in their side frames from metal fatigue.
It says no records have been kept on the age and maintenance of bogies in the fleet.
KiwiRail says it has accelerated testing of the frames and intends to finish this in four years.
However, the commission says the inspection won't be completed before 2020.
KiwiRail says it has secured funding for 300 new wagons and older ones will be retired or used on less busy lines.